Gas turbine engines typically include a compressor, one or more combustors each having a fuel injection system, and a turbine section. In an engine having a plurality of combustors, they are typically arranged in an annular array about the engine. The compressor pressurizes inlet air, which is then introduced to the combustors, where it is used to cool the combustion chamber as well to provide air for the combustion process. The hot gases resulting from the combustion process are then directed to drive a turbine. For land-based gas turbines whose primary purpose is to generate electricity, a generator is coupled to the turbine shaft such that the turbine drives the generator.
While a full load condition is the most common operating point for land-based gas turbines used for generating electricity, often times electricity demands do not require the full load of the generator, and the operator desires to operate the engine at a lower load setting, such that only the load demanded is produced, thereby saving fuel costs. Combustion systems of the prior art have been known to become unstable at lower load settings while also producing unacceptable levels of carbon monoxide and oxides of nitrogen at lower load settings, especially below 50% load. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings. However, advancements have been made with regards to fuel staging in an effort to lower emissions. For example, U.S. Pat. No. 5,551,228 discloses a method of operating a combustor involving assymetrical fuel staging within a combustor and axially staging fuel injection within a single fuel nozzle for reducing emissions. Furthermore, U.S. Pat. No. 5,924,275 discloses a method of operating a combustor that utilizes the addition of a center pilot nozzle in combination with the previously mentioned assymetrical fuel staging to provide reduced emissions at lower load conditions. While this staging method and combustor configuration is an enhancement, it is still limited in turndown capability, such that in order to achieve turndown to low part-load settings, the combustor often reverts to the higher emissions diffusion mode.
The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand. A further problem with shutting down the engine is the additional cycles that are incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life requiring premature repair or replacement at the expense of the engine operator.
What is needed is a system that can provide flame stability and low emissions benefits throughout the full operating conditions of the gas turbine engine, including a low part-load condition, such that engines can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down.